1. Field of the Invention
The present invention relates to an ink jet printing method and ink jet printing apparatus for printing an image on a print medium by using a print head.
2. Description of the Related Art
Ink jet printing apparatuses whose market has been expanded recently becomes capable of outputting not only office documents mainly on plain paper, but also images on special paper with nearly the same quality as that for silver halide photography. This achievement of high quality printing is greatly attributable to technologies of decreasing the granularity of images. The technologies include a technology of ejecting ink drops in very small size with high density from ink jet print heads, and a technology of printing an image by additionally using a light-color ink having lower density of colorant in ink than a regular one.
As for the light-color ink, light cyan and light magenta are generally used. In addition, a large number of printing apparatuses have been proposed to provide a monochrome photograph with higher image quality. Such a printing apparatus is provided with several achromatic color inks different in brightness, such as gray and light gray. Furthermore, an ink jet printing apparatus having a secondary color inks, such as red, green or blue, prepared in advance has also been provided to further widen a color reproduction range.
In this manner, in the situation that an image with high resolution is formed by using several kinds of inks, an amount of ink to be applied to a unit area is increased more than ever before. Then, if a print medium cannot absorb ink as fast as the apparatus applies ink, ink drops applied onto the same position or close positions are attracted with each other by the surface tension of the ink drops. As a result, an adverse effect of image called beading is caused.
Against the beading problem, for example, there is disclosed a technology in the specification of U.S. Pat. No. 4,999,646. In this technology, only some of print pixels being not adjacent to each other and arranged in a lattice pattern are firstly printed with ink in the first print scanning, and then residual print pixels are printed in the second print scanning. With this printing, the contact and attraction of ink drops printed in the same print scanning are avoided as much as possible, so that beading can be suppressed.
In addition, Japanese Patent Laid-open No. H06-336016 discloses a printing technology obtained by further applying the printing method disclosed in the specification of U.S. Pat. No. 4,999,646 to color printing. Japanese Patent Laid-open No. H06-336016 describes a printing method for solving various problems relating to ink jet color printing in addition to the above-described beading by preparing different mask patterns for different colors of inks in order to carry out multipass printing (multi-scans printing). In addition, Japanese Patent Laid-open No. H06-336016 discloses the configuration for making print through four passes in multipass printing by using mutually-exclusive four mask patterns so as not to overlap two or more colors of ink drops on one print pixel in a single print scanning.
However, under the recent circumstances where kinds of used inks have been rapidly increased, it is difficult to make pixels, which are printed in a single print scanning, exclusive for all kinds of inks completely. For example, if four colors of C, M, Y, and Bk are used for the multipass printing with four passes, completely exclusive mask patterns can be prepared for these four inks. However, if five or more colors of inks are used, it is not possible to prepare mask patterns giving such an exclusive relationship to all colors of inks. In the latter case, in a single print scanning, two or more colors of inks are inevitably printed on one print pixel by any means. As a result, in such a pixel, a lump (grain) of ink to easily cause beading is generated.
Against the problem, for example, Japanese Patent Laid-open No. 2006-44258 discloses mask patterns which cause dots of each color to be arranged as dispersedly as possible on a print medium in one print scanning of the multipass printing in the course of the superposition of the colors. With this, even when grains are generated by two or more colors of inks, these grains are arranged dispersedly, that is, with less low-frequency components than high-frequency components. Accordingly, the grains are more indistinctive and beading is less likely to occur.
As described above, recently, an ink jet printing apparatus has become capable of outputting a color image using a large number of ink colors, with high uniformity, by use of the mask patterns as disclosed in Japanese Patent Laid-open No. 2006-44258.
Meanwhile, recently, there is another type of ink jet printing apparatus proposed with a special mode for outputting monochrome photographs (herein after referred to as a monochromatic black mode). In this monochrome black mode, gradation of gray is reproduced by mainly achromatic color ink likes black, gray or light gray, and color inks (chromatic inks) are hardly used. In other words, the ink kind used mainly for monochrome black mode is different from that for color mode. Then, in the monochrome black mode, if the same mask patterns as those for a color mode are used, there is a case in which beading may be recognized easily.
In addition, in the monochrome black mode, even a slight error in the printing apparatus main body produces distinctive harmful effects on a formed image, although such an error does not cause a serious problem in the color mode. A mechanical error included in the printing apparatus generates non-uniformity in the dot arrangement formed by ink ejected from printing heads. The smaller size and the higher optical density a used ink drop has, the more easily such non-uniformity is recognized as density unevenness. In particular, density unevenness attributable to an error of a conveying roller conveying a print medium has been one of significant problems in the recent monochrome black mode printing. The density unevenness accompanied by such a conveying error will be described below in detail.
FIG. 1 is a schematic configuration view for illustrating each mechanism of a general serial-type ink jet printing apparatus. Driving force of a carriage motor 2 rotates a carriage belt 4, which is suspended in a tensioned state in the apparatus, and a carriage 1 connected with this carriage belt 4 moves in a main scanning direction in the figure. An unillustrated encoder sensor provided to the carriage 1 reads a pattern of a linear encoder that extends in parallel with the carriage belt 4, and thereby a current position and speed of the carriage 1 can be measured. A print head ejecting ink is mounted on the carriage 1. The print head ejects ink, according to the position information obtained by the encoder sensor and image data, while moving (scanning) in the main scanning direction.
When one print scanning by the print head is completed, a conveying roller 5 rotates along with rotation of a conveying motor 6 to convey a print medium 10, which is in contact with the conveying roller 5, by a predetermined amount in a sub-scanning direction. The driving force is transmitted from the conveying motor 6 to the conveying roller 5 via a conveying belt 8. A rotary encoder 7 having a concentric rotation axis with the conveying roller 5 is attached to the conveying roller 5. The rotation amount of the rotary encoder 7, that is, the conveying amount of the print medium by the conveying roller 5 is measured by an encoder light-receiving unit 11 fixed to the apparatus.
However, even if the rotation amount of the conveying roller 5 can be detected with relatively high-accuracy, when eccentricity, shape deformation, deflection or the like is present in the conveying roller 5, a deviation occurs between the rotation amount of the conveying roller 5 and the conveying amount of the print medium 10. Then, the deviation of the substantial conveying amount from the detected rotation amount appears as density unevenness having cycles each corresponding to one rotation of the conveying roller.
FIGS. 2A and 2B are cross-sectional views of the conveying roller for illustrating deviation of the conveying amount from the rotation amount, which is attributable to the deformation of the conveying roller 5. FIG. 2A shows a case where the cross section of the conveying roller 5 is a perfect circle, while FIG. 2B shows cases where the cross sections are two types of ellipses. If the cross-section of the conveying roller 5 is a perfect circle, the conveying amount L0 relative to the rotation amount R of the conveying roller 5 is a constant value. That is, the conveying amount relative to the rotation amount R is L0 on the side in any position of the conveying roller 5. In contrast, if the cross-section of the conveying roller 5 is an ellipse, the conveying amount relative to the rotation amount R of the conveying roller 5 varies depending on a position of the side. The figures show the positions of L1 and L2 having the largest and smallest conveying amounts relative to the same rotation amount R, respectively. In this case, the relationship of L1>L0>L2 is established. Such variations of the conveying amount appear as density unevenness if multipass printing is carried out.
FIG. 3 is a view for illustrating how variations of the above-described conveying amount cause density unevenness in a multipass printing. The left side in the figure shows phases of the conveying roller, the center shows directions in which dot landed positions are deviated in the respective phases, and the right side shows non-uniformity of dots actually printed on a print medium.
If the phase of the conveying roller 5 is L1, the conveying amount is larger than a normal amount. Accordingly, the dot is printed in a position which is advanced from an ideal position in conveying direction. In contrast, if the phase of the conveying roller 5 is L2, the conveying amount is smaller than the normal amount. Accordingly, the dot is printed in a position which is behind the ideal position in conveying direction. For this reason, even if an image having a uniform tone is printed, non-uniformity in the dot arrangement having cycles each corresponding to one rotation of the conveying roller, that is, variations of area factor appear as density unevenness in stripes. Such non-uniformity of dots arrangement and density unevenness are caused not only by the deformation of the conveying roller 5, which is shown in FIG. 2, but also by eccentricity or deflection of the conveying roller 5. The density unevenness caused as described above are referred to as conveying unevenness in the present specification.
Such conveying unevenness is not so distinctive in the color mode in which an image is printed by using many kinds of inks. However, in the monochrome black mode in which an image is printed by using mainly achromatic color inks, the conveying unevenness is particularly distinctive and thus causes an adverse effect on an image.